Method of and Apparatus for Selective Catalytic NOx Reduction in a Power Boiler

Abstract

A method of selective catalytic NOX reduction in a power boiler and a power boiler with selective catalytic NOX reduction. Fuel is combusted in a furnace of the boiler and a flue gas stream that includes NOX is generated. The flue gas stream is conducted from the furnace along a flue gas channel to a stack. The flue gas stream is cooled in a heat recovery area, including an economizer section, arranged in the flue gas channel. At least a portion of the NOX is reduced to N2 in an NOX catalyst arranged in the flue gas channel downstream of the economizer section. The flue gas is further cooled, and heated air is generated in a gas-to-air heater arranged in the flue gas channel downstream of the economizer section and upstream of the NOX catalyst. The gas-to-air heater may be a tubular air heater or a heat exchanger with a recirculating heat transfer fluid.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a method of and a system for selective catalytic NOx reduction (SCR) in a solid or a liquid hydrocarbon fuel firing power boiler. More particularly, the present invention relates to controlling the temperature of a flue gas entering an NOx catalyst of the boiler.[0003]2. Description of the Related Art[0004]Oxides of nitrogen, also known as NOX, contribute to the generation of acid rain and smog. Due to environmental regulations demanding that NOX emissions be maintained at acceptable levels, the reduction of NOX both during and after the combustion process is of a major concern in the design and operation of modern power plants.[0005]Oxides of nitrogen are a byproduct of the combustion of solid and liquid hydrocarbon fuels, such as pulverized coal or oil, and are found in two main forms. If the nitrogen originates from the combustion air, the NOX is referred to as “thermal NOX”. Thermal ...

AI Technical Summary

Benefits of technology

[0016]An object of the present invention is to provide a method of selective catalytic NOX reduction in a power boiler by which problems of the prior art described above can be minimized.

[0020]The present invention, i.e., the arranging of a gas-to-air heater upstream of the NOX catalyst to cool the flue gas, provides the advantage of rendering possible the installation of a conventional NOX catalyst using a standard catalyst material. The gas-to-air heater is preferably a tubular air heater, but may, in some cases, also be of other types of heat exchangers that transfer heat from the flue gas to combustion air of the boiler. According to another preferred embodiment of the present invention, the gas-to-air heater is a heat exchanger with a recirculating heat transfer fluid. The gas-to-air heater may, in some applications, alternatively be of another suitable type, for example, a heat pipe.

[0022]When using a method in accordance with the present invention in different load conditions of the boiler, the flow of air through the gas-to-air heater can be modulated, or shut off, to maintain a desired temperature of the flue gas entering into the catalyst. The flow of air can thus advantageously be controlled directly on the basis of the load conditions of the boiler, or on the basis of a measured temperature of the flue gas entering the NOX catalyst. Thus, the present invention provides a simple method to provide optimized operation of the catalyst in different load conditions, without, for example, a need to provide an economizer with a flue gas by-pass, or water side by-pass, for low load operation. The present invention thus provides a wide range of temperature control, without requiring any change in the flue gas or steam / water circuitry of the boiler. The invention is thus especially useful in retrofit applications, but it can be applied in new units as well, for example, to control the temperature of the flue gas entering the NOX catalyst.

Problems solved by technology

This can cause permanent catalyst activity loss.

Above 400° C., ammonia may dissociate, reducing the effectiveness of the process.

If temperatures exceed about 450° C., the catalyst activity might be permanently impaired due to sintering.

In addition to the problem addressed by U.S. Pat. No. 5,555,849, it has been observed that, especially in retrofit installations of an NOX catalyst in an existing power boiler, the flue gas temperature at the NOX catalyst may, especially at high loads, tend to be too high.

This method, however, increases the feed water temperature and, if the temperature rises close to the saturation temperature of the steam drum, it will have negative effects on the water circulation of the boiler and ultimately, reduce boiler performance.

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